Higher Order Space Harmonics Research Articles

An Endfire Periodic Leaky Wave Antenna With Wideband and High-Gain Capability by Manipulating Its Higher-Order Space Harmonics

An Endfire Periodic Leaky Wave Antenna With Wideband and High-Gain Capability by Manipulating Its Higher-Order Space Harmonics

Effects of high-order space harmonics on the operation of five-phase induction machines under unbalance

The paper addresses the effects of high-order space harmonics on the steady-state performance of five-phase induction machines operating under unbalance. We show that the airgap harmonic fields with orders higher than three, although usually disregarded, can produce a significant increase in the torque pulsation and in the Joule losses. We propose a model based on symmetric components which is used to demonstrate that the field produced by each rotor harmonic current is related to two stator sequence currents. Further, the proposed model allows determining additional losses and harmonic torques produced by airgap harmonic fields with orders higher than three, which up to now have not been addressed elsewhere. As a case study, we applied the model to two five-phase induction machines with different designs and assessed the influence of high space harmonics under operation at steady state with one open phase in the stator. The model has been validated by comparing analytical results with results obtained through finite element analysis. Finally, the model validation was also based on experimental results obtained from tests with two prototypes under many different conditions.

Parallel wave-based analog computing using metagratings.

Wave-based signal processing has witnessed a significant expansion of interest in a variety of science and engineering disciplines, as it provides new opportunities for achieving high-speed and low-power operations. Although flat optics desires integrable components to perform multiple missions, yet, the current wave-based computational metasurfaces can engineer only the spatial content of the input signal where the processed signal obeys the traditional version of Snell’s law. In this paper, we propose a multi-functional metagrating to modulate both spatial and angular properties of the input signal whereby both symmetric and asymmetric optical transfer functions are realized using high-order space harmonics. The performance of the designed compound metallic grating is validated through several investigations where closed-form expressions are suggested to extract the phase and amplitude information of the diffractive modes. Several illustrative examples are demonstrated to show that the proposed metagrating allows for simultaneous parallel analog computing tasks such as first- and second-order spatial differentiation through a single multichannel structured surface. It is anticipated that the designed platform brings a new twist to the field of optical signal processing and opens up large perspectives for simple integrated image processing systems.

A Monolayer Dually Polarized High-Order Space Harmonic Leaky-Wave SIW Antennas With Fixed and Scanning Beam Capability

In this work, the concept of high-order space harmonics (HSHs) applied to a hybrid form of nonradiative dielectric (NRD) waveguide and substrate-integrated square waveguide (SIW) involving both TE10 and TE01 modes is investigated with aim to develop multi-functional dually polarized leaky-wave antennas (DP-LWAs). A polarization-selective coupling (PSC) mechanism is used to guide the two orthogonally oriented modes in the hybrid structure to achieve a polarization diversity feature. The polarization effect on unit-cell geometrical structures is examined for different characteristics of LWA such as propagation constant behavior, leakage ratio, and scanning range variation. In addition, we demonstrate how the extrinsic characteristics of HSHs in a one-dimensional periodic LWA and its unit-cell analysis can provide design flexibility through two case studies. In the first case of demonstration, we show a dually polarized multi-beam LWA design with wide scanning capability in which we use both even and odd HSHs of n-1,-2). The second antenna design case is concerned with a DP-LWA having the capability of fixed and scanning beams operation by using a combinatory set of PSC and four HSHs of n-1,-2,-3,-4. Two DP-LWA prototypes are demonstrated as proofs of concept, and the obtained experimental results show a very good agreement with their simulation and analysis counterparts.

Higher-Order Space Harmonics in Substrate Integrated Waveguide Leaky-Wave Antennas

The concept and feasibility of deploying higher-order space harmonics (HSHs) in 1-D periodic leaky-wave antennas (1-D periodic LWAs) is proposed, studied, analyzed, and demonstrated in this article. This can lead to the creation of multiband multibeam antennas (MBMBAs) with the capability of wide scanning ranges. To show the proposed concept, the excitation and flexibility of odd (n ε [-1, -3]) and even (n ε [-2, -4]) HSHs based on the analysis of a Brillouin diagram along with the effect of unit-cell geometrical structures of substrate integrated waveguide (SIW) for different propagating media are investigated. The proposed analysis is experimentally validated by prototyping and measuring a 1-D periodic LWA in SIW technology in which two HSHs (n ε [-1, -2]) are utilized for achieving a dual-band multibeam LWA with wide scanning range capability. In addition, a triple-band multibeam LWA design based on this concept is also analyzed and demonstrated where we take advantage of the characteristics of four HSHs (n ε [-1, -2, -3, -4]). Furthermore, the band edge issue of different stop-bands of HSH is also examined, and a method is proposed and used to address this issue where we use a quarter wavelength technique to overcome the matching problem at different frequencies related to different stop-bands of various HSHs (n ε [-1, -2, -3, -4]). Another design of a SIW-based 1-D periodic LWA is also discussed to extend and generalize the proposed concept in solving the mismatch issues of HSHs focusing on broadside radiation of the n $= -2$ space harmonic. The measured results are found to agree very well with their simulation and analysis counterparts.

A Cross-Shaped 2-D Periodic Leaky-Wave Antenna

A 2-D periodic leaky-wave antenna (LWA) is a planar structure that radiates from high-order space harmonics (Floquet waves) of a leaky mode that propagates radially outward on a structure. The structure chosen here consists of a 2-D periodic cross-shaped distribution of metal patches on top of a grounded dielectric substrate and is designed to produce a highly directive elliptical beam at broadside. The beam at broadside is produced by two high-order space harmonics of a leaky TM0 surface-wave mode that propagates radially outward from a dipole source. It is shown here that the structure can be truncated to the shape of a cross, maintaining similar radiation characteristics as the full structure, while reducing undesirable grating beams. The results from theoretical analysis, full-wave simulations, and measurements for a fabricated 2-D periodic cross LWA are presented here and are found to be in good agreement.

Radiation Properties of a 2-D Periodic Leaky-Wave Antenna

A study of the radiation characteristics of a 2-D periodic leaky-wave antenna (LWA) [i.e., one that radiates from higher order space harmonics (Floquet waves) instead of the fundamental one] is presented here for the first time, and an optimization to produce maximum radiation at broadside is discussed. The structure consists of a 2-D periodic arrangement of narrow rectangular patches on a grounded dielectric substrate, excited by a simple source such as a slot in the ground plane. A 2-D periodic LWA can be designed to radiate a narrow pencil beam pattern at broadside by adjusting its geometric parameters. The main beam at broadside is narrower in the E-plane than in the H-plane, and there are also grating lobes introduced due to the periodicity. The structure has been analyzed theoretically using the periodic spectral-domain immittance (periodic SDI) method. The proposed antenna has also been simulated using both FEM and method of moments methods and a fabricated design has been measured. A good correlation is observed between the theoretical, simulated, and measured radiation patterns.

Magnetomotive force harmonic reduction techniques for fractional-slot non-overlapping winding configurations in permanent-magnet synchronous machines

Compared to conventional distributed winding configurations, the fractional-slot non-overlapping (concentrated) windings exhibit advantages such as short end-winding length, high copper packing factor (particularly with segmented stator structure), low cogging torque, good field weakening capability owing to relatively large d-axis inductance, and better fault tolerant capability due to low mutual inductance. However, one of the key problems of employing concentrated windings in Permanent-magnet Synchronous Machines(PMSMs) is the high eddy-current losses in rotor magnets and/or rotor iron due to the presence of a large number of lower and higher order space harmonics in the stator Magneto-Motive Force(MMF). These MMF harmonics also result in other undesirable effects, such as acoustic noise and vibrations, and localized core saturation which tend to reduce reluctance torque. This paper reviews the current state-of-the-art of the MMF harmonic reduction techniques for concentrated winding configurations in PMSMs, including winding split and shift, delta-star connected windings, multiple 3-phase windings, multilayer windings, uneven turn numbers, and stator flux barriers. Their concepts, advantages and disadvantages are presented and assessed.

A Nine-Phase 18-Slot 14-Pole Interior Permanent Magnet Machine With Low Space Harmonics for Electric Vehicle Applications

One of the key challenges of utilizing concentrated winding in interior permanent magnet machines (IPMs) is the high rotor eddy current losses in both magnets and rotor iron due to the presence of a large number of lower and higher order space harmonics in the stator magnetomotive force (MMF). These MMF harmonics also result in other undesirable effects, such as localized core saturation, acoustic noise, and vibrations. This paper proposes a nine-phase 18-slot 14-pole IPM machine using the multiple three-phase winding sets to reduce MMF harmonics. All the subharmonics and some of the higher order harmonics are cancelled out, while the advantages of the concentrate windings are retained. The proposed machine exhibits a high efficiency over wide torque and speed ranges. A 10-kW machine prototype is built and tested in generator mode for the experimental validation. The experimental results indicate the effectiveness of the MMF harmonics cancellation in the proposed machine.

Model-Corrected Microwave Imaging through Periodic Wall Structures

A model-based imaging framework is applied to correct the target distortion seen in microwave imaging through a periodic wall structure. In addition to propagation delays caused by the wall, it is shown that the structural periodicity induces high-order space harmonics leading to other ghost artifacts in the through-wall image. To overcome these distortions, the periodic layer Green’s function is incorporated into the forward model. A linear back-projection solution and a nonlinear minimization solution are applied to solve the inverse problem. The model-based back-projection image corrects the distortion and has higher resolution compared with free space due to the inclusion of multipath propagation through the periodic wall, but considerable sidelobe clutter is present. The nonlinear solution not only corrects target distortion without clutter but also reduces the solution to a sparse form.

Analytical Prediction of Eddy-Current Loss in Modular Tubular Permanent-Magnet Machines

The paper describes an analytical technique for predicting the eddy-current loss in the moving armature of a tubular permanent magnet machine. This loss component is usually neglected in conventional tubular permanent magnet machines since high-order time harmonics in the stator current waveform and space harmonics in the winding magnetomotive force (MMF) distribution are generally considered to be insignificant. However, a relatively new topology of tubular permanent magnet machine, sometimes referred to as "modular", has emerged in which the fundamental component of the stator MMF has fewer poles than that of the permanent-magnet armature, the thrust force being developed by the interaction between a higher order MMF harmonic and the permanent magnet field. Thus, the presence of lower and higher order space harmonics in the winding MMF distribution of a modular machine may gives rise to a significant eddy-current loss in the moving-magnet armature. An analytical model is developed to predict the eddy currents which are induced in the magnets, as well as in any electrically conducting supporting tube which may be employed, and to quantify the effectiveness of axially segmenting the magnets in reducing the eddy-current loss. The validity of the developed model, which is also applicable to conventional designs of tubular permanent-magnet machine, is verified by time-stepped transient finite-element analysis (FEA).

Analysis of a Disc-Loaded Circular Waveguide for Interaction Impedance of a Gyrotron Amplifier

A rigorous electromagnetic analysis of a circular waveguide loaded with axially periodic annular discs was developed in the fast-wave regime, considering finite axial disc thickness and taking into account the effect of higher order space harmonics in the disc-free region and higher order modal harmonics in the disc-occupied region of the structure. The quality of the disc-loaded circular waveguide was evaluated with respect to its azimuthal interaction impedance that has relevance to the gain of a gyrotron millimeter-wave amplifier (gyro-traveling-wave tube) in which such a loaded waveguide finds application as a wideband interaction structure. The results of electromagnetic analysis of the structure with respect to both the dispersion and azimuthal interaction impedance characteristics were validated against the commercially available code: high frequency structure simulator (HFSS). The analysis predicts that the value of the interaction impedance at a given frequency decreases with the increase of the disc hole radius and disc periodicity. The change of the axial disc thickness does not significantly change the value of the interaction impedance though it shifts the frequency range over which appreciable interaction impedance is obtained. Out of the three disc parameters, namely the disc hole radius, thickness and periodicity, the lattermost is most effective in controlling the value of the azimuthal interaction impedance. However, the passband of frequencies and the center frequency of the passband both decrease with the increase of the disc periodicity. Moreover, the disc periodicity that provides large azimuthal interaction impedance would in general be different from that giving the desired dispersion shape for wideband interaction in a gyro-TWT, suggesting a trade-off in the value of the disc periodicity to be chosen.

Design of Slotless-Type PMLSM for High Power Density Using Divided PM

A slotless permanent magnet linear synchronous motor (PMLSM) has a low power density due to having a large air-gap structurally. In this paper, an increase of power density of the slotless-type PMLSM is presented by inserting the core between each phase windings. The permanent magnet (PM) is divided into two pieces to reduce detent force and eliminate high-order space harmonics affecting the thrust ripples. All analysis values are calculated with two-dimensional finite-element method.